It is known that adhesives suitable for bonding a wide variety of materials such as leather, synthetic leather, rubber or plastics can be prepared from high-molecular weight hydroxy-polyurethanes. Adhesives of this kind have been described, inter alia, in German Pat. No. 1,256,822.
If such adhesives are applied without heating ("cold sealing process"), a sufficiently strong bond, especially immediately after glueing ("instant strength"), is obtained only if bonding is carried out within a certain interval of time ("open time"). For the majority of adhesives applied in this way, this "open time" is extremely brief, i.e. only a few minutes. Faulty bonds are obtained not only if the open time is exceeded but also if a shorter time is used. In order to avoid this, the adhesives are applied by a thermal activating process in which the layers of adhesive are heated to about 80.degree.C (i.e. activated), e.g., by infrared irradiation, before they are joined together. This thermal activation process, however, cannot always be applied. It firstly requires considerable expenditure relative to the apparatus to be used. Moreover, many materials, e.g. plastic sheets, are subject to deformation or damage by such treatment.
Investigations have been carried out with the object of overcoming these serious disadvantages by increasing the "open time" of hydroxy-polyurethane adhesives by means of addition of various kinds of resins. The adhesives with longer "open times" obtained in this way, however, gave rise to bonds which were not strong enough for many purposes, particularly immediately after bonding ("instant strength"). Furthermore, the addition of resins also reduced the adherence of the adhesives to many materials and substantially reduced the resistance of the bonds to plasticizers.